This application relates to the field of lipopolysaccharides (LPS) that are useful as immunoadjuvants for vaccines and vaccines comprising such immunoadjuvants.
A wide variety of antigens stimulate the production of antibodies in animals and confer protection against subsequent infection. However, some antigens stimulate only a mild or ineffective immune response while some are unable to stimulate an effective immune response.
The immunogenicity of a relatively weak antigen is often enhanced by the simultaneous administration of the antigen with an adjuvant, which is a substance that may or may not be immunogenic when administered alone, but will induce a state of mucosal and/or systemic immunity for an antigen when it is administered concurrently or proximate to the time that the antigen is administered. Unfortunately, many immunoadjuvants, such as Freund""s Complete Adjuvant, are toxic and are therefore only useful for animal research purposes, not human vaccinations.
Preferred adjuvants are substances that are not immunogenic or very toxic when administered alone, but potentiate and focus the immune response to a vaccine. Adjuvants often contain immunomodulators that induce the production of cytokine cascades and result in an augmented immune response. Examples of immunomodulators include certain water-soluble polymers, muramyl peptides, lipopolysaccharides (LPS) from gram-negative bacteria and derivatives, and cationic detergents.
The outer membrane of characterized gram-negative bacteria is composed of phospholipids, proteins, lipopolysaccharide (LPS), and if present, extracellular polysaccharide, all of which interact to form a permeability barrier towards the aqueous milieu. Of these components, it is the LPS or extracellular polysaccharide that first meets the extracellular environment and, if an LPS-layer is present must in some way accommodate its attachment.
LPS from gram-negative bacteria, also known as endotoxin, has been recognized as a potent immunomodulator. In fact, the adjuvant-active component of LPS endotoxins of gram negative bacteria has been identified as lipid A (See FIG. 1 of Chapter 21, entitled xe2x80x9cMonophosphoryl Lipid A as an Adjuvantxe2x80x9d by Ulrich and Myers, from the book Vaccine Designs: The Subunit and Adjuvant Approach, ed. By Powell and Newman, Plenum Press, New York (1995)). Unfortunately, lipid A is pyrogenic and elicits a number of undesired side effects (see, for example, the MiniReview by Raetz, in Journal of Bacteriology, pages 5745-5753 (September 1993), entitled xe2x80x9cBacterial Endotoxins: Extraordinary Lipids That Activate Eucaryotic Signal Transductionxe2x80x9d). Efforts have been made to separate the adjuvant activity from toxicity. The work of Raetz as described above relates to the structure of E. coli lipid A and of a portion of lipid A, as well as their use as an adjuvant. The structure of the lipid A endotoxin varies depending upon the type of bacteria from which it is obtained.
In general, lipid A is a disaccharide of glucosamine with two phosphate groups (at the 1xe2x80x2 and 4xe2x80x2 positions of the disaccharide) and five or six fatty acid side chains. Under mildly acidic conditions, one phosphate group can be removed leaving 4xe2x80x2-monophosphoryl lipid A (MPL). This molecule retains adjuvant activity but is much less toxic than lipid A. MPL, when formulated appropriately, can be used in humans. MPL, which is derived from Salmonella minnesota bacteria is the basis of the commercial adjuvant known as the RIBI MPL adjuvant.
Caulobacter crescentus, a stalked, gram-negative bacteria, contains an atypical LPS molecule (see, Ravenscroft et al., Journal of Bacteriology, 7595-7605 (December 1992), article entitled, xe2x80x9cMembrane Lipopolysaccharide of Caulobacter crescentusxe2x80x9d). Therein, LPS from such bacteria is described as being of a rough type in that it did not contain heterogeneous O antigen attached to the core sugars. The lipid A equivalent region and core oligosaccharide region were found to be quite distinct from all other LPS molecules. More recent analysis has shown that Caulobacter possess both rough and smooth LPS, the smooth LPS being the rough LPS-equivalent with the addition of a homopolymer antigen of perosamine (with about 40 copies of this sugar). However, nothing is known about the immunomodulating properties of Caulobacter LPS.
Although LPS from gram negative bacteria or portions thereof have been used as immunoadjuvants, it cannot be predicted whether or not an LPS is suitable for use as an adjuvant in that the LPS must not only have adjuvant properties but must also not have a toxicity level that would render the LPS non-suitable for use as an adjuvant. In addition, there is a continued need for adjuvants that are less toxic. In this respect, there is a need, inter alia, to identify naturally occurring LPS molecule(s) with adjuvant properties but with an attenuated (or negative) toxicity profile.